AJP - Gastrointestinal and Liver Physiology, Vol 251, Issue 5 665-G673, Copyright © 1986 by American Physiological Society
Taurocholate transport and Na+-K+-ATPase activity in fetal and neonatal rat liver plasma membrane vesicles
F. J. Suchy, J. C. Bucuvalas, A. L. Goodrich, M. S. Moyer and B. L. Blitzer
The ontogenesis of Na+-K+-ATPase activity and Na+-taurocholate cotransport
was studied in basolateral plasma membrane vesicles from fetal and neonatal
rat liver. Membrane vesicles from each age group were 30-fold enriched in
the basolateral marker enzyme Na+-K+-ATPase, 4- to 7-fold enriched in the
bile canalicular membrane marker enzymes alkaline phosphatase and Mg2+
ATPase, and not significantly enriched in activities of marker enzymes for
intracellular organelles. Na+-K+-ATPase activity was significantly lower in
basolateral membranes from late fetal (day 21-22) and neonatal (day 1) rat
liver. Kinetic analysis of Na+-K+-ATPase activity at various concentrations
of ATP revealed that the maximum velocity of enzyme reaction (Vmax) for
Na+-K+-ATPase was 70 and 90% of adult activity in the fetus and the
neonate, respectively. The ATP Km was significantly lower in the neonate
than the adult, suggesting a higher affinity of the neonatal enzyme for
ATP. In contrast to the early maturation of Na+-K+-ATPase, transport of
taurocholate was markedly lower in both fetal and neonatal vesicles
compared with the adult. Taurocholate uptake on day 19 of gestation did not
differ in the presence of a Na+ or K+ gradient, and uphill transport, as
indicated by an overshoot, did not occur. On day 20 taurocholate uptake was
stimulated by a Na+ compared with a K+ gradient, and accumulation of
isotope above equilibrium was demonstrated. Na+-dependent transport of
taurocholate by late fetal (day 22) and neonatal vesicles was saturable but
the Vmax at each age was significantly lower and the apparent Km higher in
developing compared with adult membrane vesicles.(ABSTRACT TRUNCATED AT 250
WORDS)
